Polyvinyl alcohol (hereinafter, may be abbreviated as “PVA”) has been known as a water soluble synthetic polymer and is superior in strength characteristics, film-forming properties, etc. to other synthetic polymers. Therefore, PVA has been widely used for applications such as paper processing, fiber processing, adhesives, stabilizers for emulsion polymerization and suspension polymerization, binders for inorganic substances and films, for example.
Exemplary intended use of the film described above may include a water soluble film. In recent years, various types of chemicals such as pesticides, laundry detergents, bleaching agents, toiletry products, industrial chemicals, etc., are being used in such a manner that the chemical is hermetically packaged in a water soluble film each in a certain equal quantity (unit packaging), and put into water in the packaged state upon use, whereby a content as well as the packaging film is dissolved or dispersed in water. Advantages of the unit packaging are that a user can use harmful chemicals without making direct contact therewith upon use, that there is no need to quantitatively determine a chemical upon use by virtue of packaging in a certain equal quantity, that no disposal is required of a container in which a chemical is packaged, and the like.
Since an increase in a degree of saponification of PVA is accompanied by an increase in crystallinity, and in turn an increase in a proportion of crystal portions which do not dissolve in cold water, for a cold water soluble film for unit packaging and the like, a partially saponified unmodified PVA is used, not PVA having a high degree of saponification, which is referred to as saponified PVA. A water soluble film formed by using the partially saponified unmodified PVA has advantageous features such as superior solubility in cold water, superior mechanical strength, and the like.
Conventional partially saponified unmodified PVA films have a disadvantage that, in a case where an alkaline or acidic substance is packaged therein, an acetoxy group remaining in the film may be saponified during storage to allow crystallization of PVA to proceed, whereby the film is likely to be insolubilized. The partially saponified unmodified PVA films have a further disadvantage that, in a case where a chlorine-containing compound such as a pesticide and a microbicide is packaged therein and then stored for a long period of time, the film may be colored and/or hardened, and water solubility thereof may decrease with time to eventually make the film insoluble or hardly soluble in water, whereby the compound is less likely to be dissolved or dispersed in water in a state of remaining packaged in the film.
In order to solve such problems, Patent Document 1 discloses a water soluble film formed from a PVA comprising an oxyalkylene group, a sulfonic acid group or a cationic group. Patent Document 2 discloses a water soluble film formed from a composition comprising: a modified PVA having a monomer unit containing a carboxy group and/or a sulfonic acid group; and a polyhydric alcohol. Patent Document 3 discloses a cold water soluble film comprising a modified PVA having a vinyl alcohol unit and a 2-acrylamide-2-methylpropanesulfonic acid unit. Patent Document 4 discloses a water soluble film formed from a modified PVA having a N-vinylamide monomer unit and a carboxy group and/or a lactone ring.
However, in recent years, in light of workability, environmental protection, etc., a water soluble film that concomitantly meets required characteristics of cold water solubility, mechanical strength and chemical resistance has been demanded. In this respect, the above described water soluble films formed from the aforementioned conventional modified PVAs have not sufficiently met these required characteristics.